I think it is hard to call a winner. But, I'm most excited about MS. I'm always most excited about MS though, as they seem to be the most likely to surprise me. This year, I'm hoping for some solid Halo 6 news.
Nintendo E3 doesn't excite me much, as they talk about too much stuff that is years away from launching.
Sony.... well, I'm sure they'll show some awesome new games. I like awesome new games. But, I feel like I know what to expect from them. So, there's less anticipation.